What are your goals here? Do you want to actually challenge the party with a "level appropriate" challenge (draining surges, daily powers, actions points like a normal encounter) or is this just an interesting bit of scenery with teeth?
I'd like it to be a level appropriate challenge. The dungeon is not densely inhabited and its inhabitants are fairly dynamic (I put notes on reinforcements in areas where they might expect nearby creatures to approach if they hear intruders, for instance).
A "room as trap" that has exits the party can readily reach won't be much of a challenge, especially if some of the party has encounter teleport powers.
This brings up an interesting point- it's possible, but not a given, that the party wizard will have arcane gate prepared. Hmm.
Why not just make it a standard combat encounter? Or have the Gibbering Cavern close the exits after 3 or more of the party enter the room?
The real challenge is getting across the room rather than fighting something. My hope is to make this an interesting but fairly quick encounter, as the pcs spend their move actions to slog across the room and then experiment with ways to guard or help each other. Also, I am hoping that this will be quicker than a full combat encounter. (Not sure about that one!)
If this is intended to constitute an entire encounter on it's own I would include a bit more background/description of the room and some more countermeasures at the minimum... Why is there Far Realms influence? And why in this particular room?
For the record, this is the other text describing this encounter:
The Bile Mountain Casino and Resort said:
The Hazard activates as soon as a non-aberrant creature enters the room.
This room's walls, floor and ceiling are covered with small muttering mouths. The strange odor is different in here- unpleasant, like some strange burnt work of alchemy.
Note that characters only earn xp for this encounter if they pass through the room.
So there isn't much in the encounter itself. However, the entire level that this is on is Far Realms-corrupted due to the influence of some of the BBEGs in the "old days". Distance and direction are distorted in places (you go out the southwest exit of this chamber and come in the next room in the west wall; some passages have an extra 100' of passageway compressed into one square of map; etc). The room through which the pcs are most likely to enter the level has an old warning left by a different group of adventurers chalked on the wall, so they'll have a blatant, can't miss it clue that things are messed up. Every room on the level has some weird, aberrant, alien stuff going on- whether it's the gibbering cavern or a room where the pool of dark liquid on the floor keeps dripping up onto the ceiling. But most of the level, while disquieting, isn't actually too dangerous. The gibbering cavern is one of the exceptions.
For example in describing the room, what cues do the players have not to drop their harder hitting area spells to blast way this giant mimic (for all they know)? What cues do they have that intimidating this thing would be effective?
None, but with their first attack I will make it explicit that attacking isn't doing anything. The deafness/plug ears/make lots of noise options seem fairly obvious to me; the intimidate the room/fear effect option isn't obvious at all, but I'd probably let a pc who asked to make an Insight check figure this one out.
Was it Matthew Sernett's trap article or DMG2 where the author pointed out that solo traps usually are better served by skill challenges? While I don't necessarily agree with that ascertain, I do think the spirit of it is spot on: solo encounters require more narrative (background story, interesting developments, unique rules, special countermeasures, etc).
EDIT: Further thoughts... Make it a "dominate and..." effect with something unique to the Far Realm theme like "the room can make it's psychic attacks thru dominated PCs." So domination is more like assimilation. Also introduce a countermeasure our leader is likely to employ that restores a PC to him or herself (eg. Diplomacy/Insight to grant a save).
Good suggestion! But note that the Bluff/Diplomacy/Intimidate countermeasure can already grant an ally a save against the effects of the gibbering (including the domination).
